Physicians have long realized that patients who are unable to clear their lungs of secretions are at a much greater risk of developing pneumonia or atelectasis. These patients often die prematurely of respiratory failure. The inability to expectorate secretions is considered, by physicians, a prime contributor to the demise of patients suffering from neuromuscular disorders, cystic fibrosis, bronchiectasis, pneumonia and obstructive lung diseases. In addition, the excessive morbidity and mortality of critically ill patients requiring mechanical ventilation or patients recovering from abdominal or thoracic surgery has been attributed in part to their inability to effectively expectorate airway secretions. Patients with spinal cord injuries are also at a greater risk of respiratory failure and death because of the effect of the injury on inspiratory and/or expiratory muscles. Such patients are often unable to generate adequately high pleural pressures during coughing to achieve expectoration of secretions. Secretions that are not removed from the respiratory tract in a natural manner not only obstruct the airways thereby interfering with breathing but also become contaminated with pathogenic bacteria. This leads to an inflammatory reaction which may damage the lungs irreversibly.
As secretions can be moved from lower parts of the respiratory tract towards the upper airways (upper bronchi, trachea and mouth) by a high expiratory flow rate of air from the lungs, research work has been undertaken to induce enhanced air flow rates from the patient's lungs. Certain methods typically employ a cyclic manoeuvre in which the lungs of the patient are over pressured prior to the patient exhaling into a source of vacuum. This approach is termed "exsufflation with negative pressure" (E.W.N.P.). Note, pressures denoted negative and positive refer respectively to sub-atmospheric and super-atmospheric pressure.
As defined in Blakiston's Gould Medical Dictionary, Fourth Edition, McGraw-Hill, 1979, exsufflation means "forcible expiration; forcible expulsion of air from lungs by a mechanical apparatus", and exsufflator is defined as "an apparatus that can mimic the effect on the bronchial tree of a vigorous cough, by the sudden production of negative pressure". Urdang Dictionary of Current Medical Terms, John Viley & Sons 1981, defines exsufflation as "the forcible removal of secretions from the air passages by some form of suction apparatus". Taber's Cyclopedic Medical Dictionary, F.A. Davis Company, Edition 15, 1985, explains exsufflation as "forceful expulsion of air from a cavity by artificial means, such as use of a mechanical exsufflator". It can be seen that the definitions are not quite consistent and the term "exsufflation" has been used in the literature with certain liberty. Although having certain advantages, E.W.N.P. also has negative effects, particularly on patients with pulmonary problems. These aspects were discussed in a paper by G.J. Beck and L.A. Scarrone, Physiological effects of E.W.N.P., Diseases of The Chest, January 1956, Vol. 29, pp. 80-95. That paper describes a breathing control procedure in which the positive pressure in inspiration was built up to 40 mm Hg above atmospheric in two seconds, then a pressure drop to 40 mm Hg negative pressure would occur in 0.04 sec. The negative pressure was maintained for approximately 1.5 sec.
To improve airflow during a natural cough humans contract their expiratory muscles, raising the pleural pressure. The pleural pressure changes are not only transmitted to the alveoli, but also to the airways. Thus, if a person tries to "force" his/her expiration to achieve a relatively high air flow from their lungs, the intrathoracic pressures combined with the low static pressure within the respiratory tract resulting from the increase in expiratory flow rate tend to close, prematurely, certain parts of the respiratory tract (bronchioles, larger bronchi, trachea). The collapse of trachea and/or bronchial tubes (dynamic compression of the airways), while temporarily stopping the expiratory flow or reducing it significantly, creates a serious problem in attempting to assist the patient in breathing.
In particular, with suction applied, researchers have found that the flow of air from the lungs during the application of negative pressure (alternatively with or without positive pressure) cannot be sustained for the full lung volume, or full vital capacity (FVC); instead, the collapse of the airways or some part thereof causes discomfort of the patient. If negative pressure is still applied to the patient's respiratory tract after a partial collapse of the airways has taken place, the patient will experience an unpleasant feeling resultant from the collapse of the cheeks and/or soft palate. This sensation has been dubbed the "alligator effect". To alleviate the patient discomfort the operator must disconnect the vacuum and allow the airways to reopen and the cheeks and soft palate to move to their normal positions. This reopening of the airways and re-positioning of the cheeks and soft palate may be aided with the application of a positive pressure into the mouth. This alternation of negative/positive pressure, or negative/atmospheric pressure has so far been left to the operator's judgment, or based on average person's parameters.
Knudson, R.J. et al., Contribution of airway collapse to supramaximal expiratory flows, Journal of Applied Physiology, Vol. 36, No. 6, June 1974, describe a flowmeter to measure rapid expiratory flow events. Controlled by the patient's pleural pressure, rapid opening and closing of a valve connected with the patient's respiratory tract produced flow rate transients of short duration which exceeded the normal flow rates. The authors found that supramaximal flows generated as a result of very fast valve opening time, below 10 msec, are relatively high although of very short duration.
A Russian Author's Certificate No. 762,890 published Sep. 15, 1980 discloses a device to induce an artificial cough. The invention recognizes the fact that the displacement of secretions from the lower parts of the respiratory tract to the upper parts--bronchi, trachea, mouth--is promoted by a fast flow of air from the lungs during exhalation. The device comprises a mask, a vacuum volume connected to a membrane switching mechanism having an inlet valve connected to a rarefaction source, and an outlet valve connected to the mask. The switching mechanism consists of three membranes and a nozzle. A coughing impulse is created by connecting the rarefaction source with the lungs. The specification is silent on the timing of the alternating vacuum phases.
While the above-described proposals are useful in removing secretions from the respiratory tract, they do not fully solve the problem of expectoration of secretions in weakened individuals without causing their discomfort.